Equipment and techniques for drilling oil and gas wells in deep water are well established. The typical practice, as shown in FIG. 1, utilizes floating drilling rig 1, from which drill string 2 is extended to the sea floor. At depths greater than 2000 feet, the vessel is typically dynamically positioned (i.e., the vessel's propulsion is controlled by a computer system to keep the vessel on location without the use of anchors). Drill bit 3 is attached to the lower end of drill string 2, and the drill string/bit combination is driven in rotation by top-drive rotary unit 4 from rig 1 to drill the well bore into the ocean floor. Once the initial well bore is established, blowout preventer 5 is lowered to the sea floor to sit above the well. Floating drilling rig 1 and blowout preventer 5 are connected by riser pipe 6 extending from beneath drill floor 7 to the top of blowout preventer 5 on the sea floor. The riser is thus fixed to blowout preventer 5 at the lower end while the upper end must move both horizontally and vertically with the motions of the floating platform. To accommodate platform movement, the riser is connected to blowout preventer 5 at the bottom through flexjoint 8 which allows for riser tilt angles with respect to blowout preventer 5 of up to a maximum of about 10.degree., and to floating platform drill floor 7 at the surface of the sea through a slipjoint/tensioner combination which allows extension and contraction of riser pipe 6 length to accommodate vessel horizontal and vertical movement. The slipjoint is made up of two parts, inner barrel 9 which is attached to the underside of the floating platform drill floor 7, and an outer barrel 10 that is attached to riser pipe 6. Inner barrel 9 slides inside the outer barrel 10 with typical slipjoint strokes being in the order of 20-30 feet. Riser pipe 6 is kept under tension by a series of hydraulic/pneumatic rams known as "riser tensioners" 11 which are located around the slipjoint in a so called "moonpool area" 12 beneath the floating platform drill floor 7. (Moonpool area 12 is a hole in the center of the vessel through which drilling equipment is lowered to and retrieved from the sea floor.) A wire line and two sheaves (one sheave 13 mounted on a tensioner piston rod attached to riser tensioner 11 and the other sheave 14 attached to rig 1). Line 15 is wound through the sheaves with one end attached to slipjoint outer barrel 10 and the other end to the rig 1. This allows the tensioners 11 to maintain a constant (operator selected) tension on the riser pipe 6 while the piston/sheaves move up and down with the vessel/slipjoint motion.
Once blowout preventer 5 is landed on the bottom and riser 6 has been tensioned, drilling proceeds with drill string 2 lowered down and into the sea floor through riser 6 and blowout preventer 5. Vertical rig motion is uncoupled from the drill string 2 by heave compensator 16 located in the derrick above the top drive. During drilling, the well bore is lubricated by drilling "mud" which is pumped down the center of the drill string 2 and out through drill bit 3 where it is returned to the surface through riser 6. Large "mud pumps" on the drilling rig 1 continuously circulate the drilling mud.
Flexjoint 8 at the bottom and the slipjoint at the top allow for limited vessel horizontal displacement, but if the vessel displacement is too great (or anticipated to be so), the vessel/riser must be disconnected from blowout preventer 5 at the bottom before the displacement limit is reached. To allow this disconnection, blowout preventer 5 is made up of two sections, a lower section referred to as "the stack" 17 and an upper section called the lower marine riser package 18. When a disconnection occurs, the stack 17 remains on the bottom and the lower marine riser package 18 remains connected to riser 6.
Reasons for having to disconnect are due to either anticipated heavy weather (rig motions become too large in response to high winds and seas) or the inability of the vessel to remain on location over the well site (due to heavy weather, equipment malfunctions, and operator error). If sufficient time is available, the drill string is withdrawn to the rig before disconnecting the riser. However, equipment malfunctions and/or operator errors which result in rapidly degraded station keeping capability of the vessel can create a situation whereby an "emergency disconnect" is necessary. In this case there is not time to withdraw the drill string and an automated set of blowout preventer functions is commanded from the surface which result in drill string 2 being sheared off inside blowout preventer 5 by large hydraulic rams, the oil or gas well being closed off by additional blowout preventer rams, and lower marine riser package 18 being disconnected from the stack 17. Time to complete an emergency disconnect is in the order of 35-45 seconds (note that for each vessel, the precise time to carry out the automatic disconnect sequence is determined as part of periodic testing).
In the above instances, if the riser is not disconnected in time, the drilling equipment can be seriously damaged and in some cases a serious accident or oil spill could result. Conversely, if an emergency disconnect is made prematurely, in a situation where the displacement limit would not have been reached, the resultant monetary expenditure due to possible mud loss and the time required to reconnect (possibly having to retrieve the sheared portion of drill string 2 from within the well) and re-commence drilling operations could easily be in the order of hundreds of thousands of dollars.
Historically (due to offshore drilling starting in shallow water (500-1000 ft.) and the gradual movement from shallow to deep water drilling), the bottom flexjoint angle has been the sole parameter monitored to decide when to initiate an emergency disconnect. The angle is monitored through the use of dual axis tile sensors 19 and 21, the first located on riser 6 above the slipjoint, the other on the lower marine riser package. (Note that in some cases a third tilt sensor is used to monitor the top riser angle.) With a 10.degree. flexjoint angle being the physical limit, alarms were typically set at 5.degree. (the so called "yellow" warning alarm) and at 8.degree. (the "red" emergency disconnect alarm). With increasing water depth, the traditional practice of monitoring the flexjoint angle and top riser angle to insure a maximum value of 10.degree. begins to lose its effectiveness. There comes a point at which the slipjoint and/or tensioners will strokeout before the 10.degree. flexjoint angle is reached. This has been dealt with up until now by reducing the yellow and red flexjoint alarm limits, but this is an over simplified approach which does not take into account the real factors affecting the efficient operation of the drill string and riser pipe at increased depth.
Systems for position monitoring are currently available. One such system is described in U.S. Pat. No. 4,205,379, issued May 27, 1980 to Fox, et al which is incorporated herein by reference for background details concerning techniques for deep offshore wells. FIG. 1 shows a prior art marine platform positioning system. Fox et al uses measured angles of the riser or drill string at the platform and at the floor and a special algorithm to determine the proper correction of the platforms horizontal position.
Systems such as described by Fox are helpful, but as the depth increases, more precise information is needed to avoid problems in many operating situations.
What is needed is a better system for monitoring disconnect parameters for very deep offshore wells.